Field of the Invention
The present invention relates to a method for reducing an exhaust gas of a mild hybrid system. More particularly, it relates to a method for reducing an exhaust gas of a mild hybrid system, which can reduce an exhaust gas generated in sudden acceleration of a vehicle equipped with an electric supercharger as a supercharger for pressurizing air and supplying the pressurized air to an engine.
Description of Related Art
In general, after introducing external air, a vehicle mixes the air with a fuel to supply the mixture to an engine, and the engine combusts the mixture of the fuel and the air to obtain power necessary to drive the vehicle.
In a process of generating the power by driving of the engine, a desired output and combustion efficiency of the engine is obtained only when the external air is to be sufficiently supplied for combustion. Accordingly, in order to improve the combustion efficiency of the engine and increase the output of the engine, a supercharger or a turbocharger for pressurizing and supplying combustion air has been applied to the vehicle.
The turbocharger has a structure compressing air supplied to the engine by using the pressure of an exhaust gas exhausted from the engine. More specifically, the turbocharger has a structure rotating a turbine by using the pressure of an exhaust gas exhausted from the engine and compressing external air introduced through an intake filter as a compressor coaxially connected to the turbine, thereby supplying the compressed air to a combustion chamber of the engine.
However, there is a limitation in compressing intake air with only the pressure of an exhaust gas and supplying the compressed intake air to the engine according to a driving situation of the vehicle.
For example, when an engine load is increased and the amount of acceleration is rapidly increased due to transient operation in driving of the vehicle, the supercharging of the turbocharger should be made according to the transient operation. However, the turbocharger is operated only when the turbine is rotated by the pressure of the exhaust gas, and therefore, a turbo lag (turbo response delay) phenomenon occurs.
Accordingly, it is difficult for the turbocharger to achieve a desired supercharging function as compared with the increase in the engine load and the rapid acceleration. Hence, a supercharging pressure and an air amount for combustion of the engine are deficient as compared with target amounts, and therefore, the combustion of the engine deteriorates. Also, as an air amount for exhaust gas recirculation (EGR) control is deficient as compared with a target amount, an EGR valve for supplying the exhaust gas to the front end of the turbocharger is closed, and hence the recirculation of the exhaust gas is not made. Accordingly, the amount of the exhaust gas increases, and the amount of nitrogen oxide contained in the exhaust gas is maximally generated. Therefore, the air pollution becomes severe.
The information disclosed in this background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.